Alif Ahmed / Mbed OS digitalInPolling_sample_program

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main.cpp@4:82f7ccb294d3, 2020-01-30 (annotated)

Committer:
WilliamMarshQMUL
Date:
Thu Jan 30 08:04:56 2020 +0000
Revision:
4:82f7ccb294d3
Parent:
3:8d87cbabe37e
Child:
5:f95f17441dab
Replace wait with sleep_for

Who changed what in which revision?

UserRevisionLine numberNew contents of line
WilliamMarshQMUL 0:a66a8cb0012c 1 #include "mbed.h"
WilliamMarshQMUL 0:a66a8cb0012c 2
WilliamMarshQMUL 0:a66a8cb0012c 3 // Labs 2: Example program for polling an input
WilliamMarshQMUL 0:a66a8cb0012c 4 // --------------------------------------------
WilliamMarshQMUL 0:a66a8cb0012c 5 // The program uses a thread to poll a digital input
WilliamMarshQMUL 0:a66a8cb0012c 6 // - The thread monitors the position of the button
WilliamMarshQMUL 0:a66a8cb0012c 7 // - When the button transitions up and down, a press event is signaled
WilliamMarshQMUL 0:a66a8cb0012c 8 // - Button bounce is guarded against
WilliamMarshQMUL 2:cd1fe8c29793 9 // A second thread (the default one) checks for the press event and toggles the LED
WilliamMarshQMUL 0:a66a8cb0012c 10
WilliamMarshQMUL 2:cd1fe8c29793 11 DigitalIn b1(PTD0, PullUp);
WilliamMarshQMUL 2:cd1fe8c29793 12 DigitalOut led(LED1);
WilliamMarshQMUL 0:a66a8cb0012c 13
WilliamMarshQMUL 0:a66a8cb0012c 14 Thread pollT ; // thread to poll
WilliamMarshQMUL 0:a66a8cb0012c 15 volatile int pressEvent = 0 ; // Variabe set by the polling thread
WilliamMarshQMUL 4:82f7ccb294d3 16 // Sharing a global variable between two threads is not safe in general
WilliamMarshQMUL 4:82f7ccb294d3 17 // but it can be ok if a) update is atomic and b) only one thread writes
WilliamMarshQMUL 0:a66a8cb0012c 18
WilliamMarshQMUL 0:a66a8cb0012c 19 enum buttonPos { up, down, bounce }; // Button positions
WilliamMarshQMUL 0:a66a8cb0012c 20 void polling() {
WilliamMarshQMUL 0:a66a8cb0012c 21 buttonPos pos = up ;
WilliamMarshQMUL 0:a66a8cb0012c 22 int bcounter = 0 ;
WilliamMarshQMUL 0:a66a8cb0012c 23 while (true) {
WilliamMarshQMUL 4:82f7ccb294d3 24 ThisThread::sleep_for(30) ; // poll every 30ms
WilliamMarshQMUL 0:a66a8cb0012c 25 switch (pos) {
WilliamMarshQMUL 0:a66a8cb0012c 26 case up :
WilliamMarshQMUL 2:cd1fe8c29793 27 if (!b1.read()) { // now down
WilliamMarshQMUL 0:a66a8cb0012c 28 pressEvent = 1 ; // transition occurred
WilliamMarshQMUL 0:a66a8cb0012c 29 pos = down ;
WilliamMarshQMUL 0:a66a8cb0012c 30 }
WilliamMarshQMUL 0:a66a8cb0012c 31 break ;
WilliamMarshQMUL 0:a66a8cb0012c 32 case down :
WilliamMarshQMUL 2:cd1fe8c29793 33 if (b1 == 1) { // no longer down
WilliamMarshQMUL 0:a66a8cb0012c 34 bcounter = 3 ; // wait four cycles
WilliamMarshQMUL 0:a66a8cb0012c 35 pos = bounce ;
WilliamMarshQMUL 0:a66a8cb0012c 36 }
WilliamMarshQMUL 0:a66a8cb0012c 37 break ;
WilliamMarshQMUL 0:a66a8cb0012c 38 case bounce :
WilliamMarshQMUL 2:cd1fe8c29793 39 if (b1 == 0) { // down again - button has bounced
WilliamMarshQMUL 0:a66a8cb0012c 40 pos = down ; // no event
WilliamMarshQMUL 0:a66a8cb0012c 41 } else if (bcounter == 0) {
WilliamMarshQMUL 0:a66a8cb0012c 42 pos = up ; // delay passed - reset to up
WilliamMarshQMUL 0:a66a8cb0012c 43 } else {
WilliamMarshQMUL 0:a66a8cb0012c 44 bcounter-- ; // continue waiting
WilliamMarshQMUL 0:a66a8cb0012c 45 }
WilliamMarshQMUL 0:a66a8cb0012c 46 break ;
WilliamMarshQMUL 0:a66a8cb0012c 47 }
WilliamMarshQMUL 0:a66a8cb0012c 48 }
WilliamMarshQMUL 0:a66a8cb0012c 49 }
WilliamMarshQMUL 0:a66a8cb0012c 50
WilliamMarshQMUL 2:cd1fe8c29793 51 /* ---- Main function (default thread) ----
WilliamMarshQMUL 2:cd1fe8c29793 52 Note that if this thread completes, nothing else works
WilliamMarshQMUL 2:cd1fe8c29793 53 */
WilliamMarshQMUL 2:cd1fe8c29793 54 int main() {
WilliamMarshQMUL 2:cd1fe8c29793 55 led = 1 ; // Initially off
WilliamMarshQMUL 2:cd1fe8c29793 56 pollT.start(callback(polling));
WilliamMarshQMUL 2:cd1fe8c29793 57
WilliamMarshQMUL 0:a66a8cb0012c 58 while(true) {
WilliamMarshQMUL 0:a66a8cb0012c 59 if (pressEvent) {
WilliamMarshQMUL 0:a66a8cb0012c 60 pressEvent = 0 ; // clear the event variable
WilliamMarshQMUL 0:a66a8cb0012c 61 led = !led ;
WilliamMarshQMUL 0:a66a8cb0012c 62 }
WilliamMarshQMUL 4:82f7ccb294d3 63 ThisThread::sleep_for(100) ; // delay for 100ms
WilliamMarshQMUL 0:a66a8cb0012c 64 }
WilliamMarshQMUL 0:a66a8cb0012c 65 }